Carbon nanomaterials for the removal and recovery of metal ions from aqueous solutions

Show simple item record Pillay, Kriveshini 2011-07-25T10:48:33Z 2011-07-25T10:48:33Z 2011-07-25
dc.description.abstract Carbon nanomaterials are a group of materials which have been gaining increasing recognition for their applications in environmental remediation. Studies on the use of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) for the remediation of various contaminants including metal ions like Cr(VI) and Hg(II) have already been undertaken. However, these studies have mostly focussed on the uptake of parts per million (ppm) levels of contaminants. The uptake of parts per billion (ppb) levels of such contaminants have not been extensively examined. Studies on the effect of sulphur doping on the uptake of Hg have not been undertaken. Competitive adsorption studies in which competing cations and anions are present have also been scarce. The effect of surface modifications such as acid and base-treatment and the effect of nitrogen doping have not been extensively studied. This study was therefore undertaken to address the above issues. It was found that unfunctionalised MWCNTs could adsorb 98% of a 100 ppb Cr(VI) solution. These materials had superior adsorption capabilities to that of activated carbon and functionalised MWCNTs. The difference in adsorption capabilities of these materials was attributed to the differences in points of zero charge (pHpzc). The Langmuir and Freundlich adsorption isotherm models were also used to describe the adsorption process. Sulphur-containing MWCNTs (S-MWCNTs) showed the highest uptake capacities for both 100 ppb solutions of Hg(II) and organic forms of mercury respectively. This was attributed mainly due to mercury’s high affinity for sulphur and evidence of a chemisorption process was presented. The uptake capacity of sulphur-containing activated carbon (S-AC) was inferior to that of the S-MWCNTs but this was attributed mainly to a difference in sulphur content. The only advantage that S-MWCNTs presented over S-AC was a greater selectivity in the presence of SO2. The S-MWCNTs were also highly selective to the uptake of Hg in the presence of competing cations and in a chlor-alkali effluent where a complex chemical matrix was noted. The Freundlich adsorption isotherm model best described the uptake of Hg. Both Cr(VI) and Hg were efficiently desorbed in 0,1 M NaOH and 0,5% thiourea in 0,05 M HCl solutions respectively. This implied that the MWCNTs could be reused and regenerated and this could address cost issues. Results from the surface modification studies showed that acid-treatment (both strong and weak) resulted in oxygen-containing functional groups which lowered the point of zero charge of the MWCNTs thereby rendering these suitable for cation uptake. The effect of base-treatment depended on the type of base used. The strong base KOH had a similar effect to that of the acid-treated MWCNTs. The weak base NH3 on the other hand resulted in the presence of quartenary nitrogen which increased the point of zero charge and made the MWCNTs more suitable for anion uptake. Similar observations were made for nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) and nitrogen doped carbon spheres (N-CSs) . The effect of nitrogen doping does however depend on the form in which nitrogen is present. In this case the predominant form of nitrogen was quartenary nitrogen. Carbon nanomaterials have therefore demonstrated a great ability to extract a variety of anions and cations from aqueous solution and thus show potential for industrial applications especially since they show superior adsorption capabilities to that of activated carbon. The selectivity of the unfunctionalised MWCNTs and MWCNTs treated with acid and base was however, poor when competing anions and cations were present. This suggested that the selectivity of these materials needs to be improved upon in further studies before these are used in industrial applications. en_US
dc.language.iso en en_US
dc.title Carbon nanomaterials for the removal and recovery of metal ions from aqueous solutions en_US
dc.type Thesis en_US

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